The ability to quickly and cleanly sever tubular members, such as well casings that are deep underground, is an essential step during well maintenance and salvage operations. Typically, the industry relies on mechanical or explosive devices to perform such cutting. One type of explosive device, that is often used, is a shaped charge explosive cutter, which provides a simple, fast, and inexpensive method by which to sever pipes within a wellbore. During typical operations, shaped charge explosive cutters are lowered to a selected depth within a well, using a wireline, at which time they are detonated, producing pressure and/or molten materials that cut through the casing.
A typical shaped charge tubular cutting device contains two circular layers of explosive material, each having a truncated cone shape. Outlining the sloped side faces of the explosive circular layers are thin metal rings, called half-liners. These two components are joined together, apex-to-apex, forming a shaped charge assembly having a liner with a V-shaped cross section. The shape charge assembly is sandwiched between two end plates, typically made from steel. Lastly, the six elements (two layers of explosive, two half-liners, and two end plates) are aligned coaxially and enclosed within a cylindrical housing, in the recited order.
The end plates contain an opening along the central axis to provide a pathway for an explosive detonator to be placed adjacent to the top circular layer of explosive material. The two circular layers of explosive material may also contain an opening along their central axes, providing a space for an explosive detonator to be placed between the circular layers of explosive material.
After the shaped charge tool is assembled, it is lowered into the tubular member. For optimal effectiveness, the circular shaped charges within the tool must be aligned at a substantially perpendicular angle, relative to the tubular wall. Following the placement of the shaped charge tool at the proper location within the tubular member, the shaped charge is detonated.
Once the charge is detonated, a shock wave propagates radially along the transverse plane between the circular half charges and collides with the V-shaped liner, forcing the two liner surfaces together at high speeds. The resulting impact between the two liner surfaces results in extreme pressure being generated. At these high pressures, the metal liner exhibits plastic and/or fluid-like characteristics. While the expanding shock wave folds the metal liner together into a disc shape, the shock wave continues to advance radially along the transverse plane, pushing and accelerating the liner material to flow radially along the transverse plane at extreme velocities, forming a jet of liner material able to cut through the tubular member.
Traditional fabrication procedures for circular shaped charge tools include independent fabrication of the half-liner pieces, each having a truncated cone shape, with an open base and apex surface. The circular explosive discs can be formed using half-liners as the outside wall portions of the mold. The apex surface of the explosive disc is formed against the bottom of a flat mold, the explosive material is packed into the area between the mold and the half-liner, then a top mold plate is pressed against the explosive material, solidifying and bonding the material with the half-liner. This method forms a circular disc of explosive material, with the half-liner outlining the radial walls of the disc. A unified disc of explosive material bonded with a half-liner is called a half-charge. To form the shaped charge tool, two half charges are placed apex-to-apex, in a cylindrical housing between two steel plates, as described above.
Another traditional fabrication procedure for making circular shaped charge tools includes forming the circular explosive disc without half-liners outlining the radial walls of the explosive disc. The explosive charge material is formed into a truncated cone shape by using a mold to shape every surface of the charge, including the outside wall surface. This fabrication technique results in the half-liner and the explosive material disc being separate components, which must later be arranged within a cylindrical housing.
A shaped charge assembly comprising two or more explosive charge members, such as half-charges, results in small areas of separation between such members, which allow for overrunning of the detonating shock front. As the shock wave propagates radially from the central detonation point, the areas of separation between explosive charge portions allow a shock front to travel through the empty area at faster velocities than through areas containing explosive material. This shock front collides with the center of the liner, along the transverse plane between the half-charges, before the main shock wave impacts the rest of the liner. Such non-uniform collision can cause the liner jet to scatter or to be deformed excessively at the center, as opposed to a desired compact liner jet moving in the radial direction.
In another traditional manufacturing process, the circular explosive discs are fabricated in several pieces, such as in quarters. These quarters are then arranged to form circular explosive discs when assembling the components in a cylindrical housing. A half charge may comprise four or more segments (e.g., wedge-shaped segments that together form a circle). Such an arrangement creates multiple areas of separation between the segments of explosive material, subject to the same difficulties present when using half-charges: as the shock wave propagates, the areas of separation provide empty pathways through which the shock front travels at faster velocities than through areas containing explosive material. This overrunning shock front collides with the liner in certain areas before the main shock wave impacts the rest of the liner, resulting in a non-uniform collision, causing the liner to be deformed and/or scattered excessively at points along the areas between adjacent segments of explosive material.
In addition to configurations that include multiple segments of explosive material, the space between two half liners, or between other configurations involving multiple liner pieces, also contributes to improper liner jet formation. As the shock wave impacts and collapses the V-shaped liner, the small space between the two half liners, or between other portions, allows the passage of expanding gasses into the standoff space, disrupting the formation of a uniform jet or slug. A deformed or non-symmetrical jet or slug reduces the penetrating efficiency of the shaped charge cutting tool.
Conventional tubular cutter tools typically incorporate explosive material sections that are relatively thick throughout (i.e. from the detonator to the liner). Other designs incorporate top and bottom housing plate surfaces that are sloped or that contain sharp edges or angles. Uneven plate surfaces can cause shock wave deflections in various directions within a thick layer of explosive material. Shock wave deflections may cause shock front overrunning along the path of deflection through the explosive material. This results in certain parts of the shock wave striking an area of the liner along the vertical plane before the main shock wave strikes the rest of the liner. A non-symmetrical collision causes the liner to be deformed unevenly, resulting in a non-symmetrical liner jet formation, thus reducing the effective penetration capabilities of the cutter and causing uneven severing of a tubular member. Shock wave deflections may also cause shock wave cross propagation, which occurs when shock waves having opposite directional component vectors collide and interfere with one another. Such shock wave collisions result in explosive energy loss, which also reduces the effective penetration capabilities of the cutter.
An energy loss due to separation between the upper and lower end plates prior to jet formation is also a common problem with many conventional shaped charge cutting tools. As the explosive material is detonated, explosive energy is released in all directions. If the area between the end plates expands prior to jet formation, energy is lost when deforming and accelerating these end plates, resulting in less energy available to be utilized toward liner jet formation.
Over years of experimentation, shaped charge cutter technology has developed extensively. Certain physical characteristics of shaped charge elements and certain relationships between those elements have been revealed as significant, even though prior understanding of the technology labeled them as unimportant. Departures from conventional methods, that may have previously been thought of as minute or insignificant, have led to unpredictable results, significant performance improvements, and reductions in material and fabrication costs.
A need exists for a shaped charge tubular cutter tool that overcomes the deficiencies of conventional cutters by preventing detonation front overrunning along the transverse plane between adjoining partial charges and between adjoining explosive material segments.
A further need exists for a shaped charge cutter tool that eliminates internal shock wave deflections, which can result in shock front overrunning and shock wave cross propagation.
A need also exists for a casing cutter tool that is highly efficient, utilizing more explosive energy into the cutting action than standard explosive tubular cutters.